linux-kernel - Re: [RFC, drm-misc-next v4 0/9] PCI/VGA: Allowing the user to select the primary video adapter at boot time

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Date:   Wed, 6 Sep 2023 08:47:13 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Sui Jingfeng <sui.jingfeng@...ux.dev>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     Sui Jingfeng <suijingfeng@...ngson.cn>,
        nouveau@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-pci@...r.kernel.org,
        Thomas Zimmermann <tzimmermann@...e.de>
Subject: Re: [RFC, drm-misc-next v4 0/9] PCI/VGA: Allowing the user to select
 the primary video adapter at boot time

Am 05.09.23 um 16:28 schrieb Sui Jingfeng:
> Hi,
>
> On 2023/9/5 21:28, Christian König wrote:
>>>>
>>>> 2) Typically, those non-86 machines don't have a good UEFI firmware
>>>>     support, which doesn't support select primary GPU as firmware 
>>>> stage.
>>>>     Even on x86, there are old UEFI firmwares which already made 
>>>> undesired
>>>>     decision for you.
>>>>
>>>> 3) This series is attempt to solve the remain problems at the 
>>>> driver level,
>>>>     while another series[1] of me is target to solve the majority 
>>>> of the
>>>>     problems at device level.
>>>>
>>>> Tested (limited) on x86 with four video card mounted, Intel UHD 
>>>> Graphics
>>>> 630 is the default boot VGA, successfully override by ast2400 with
>>>> ast.modeset=10 append at the kernel cmd line.
>>> The value 10 is incredibly arbitrary, and multiplied as a magic number
>>> all over the place.
>>
>> +1 
>
>
> This is the exact reason why I made this series as RFC, because this 
> is a open-ended problem.
> The choices of 3,4,5,6,7,8 and 9 are as arbitrary as the number of 
> '10'. '1' and '2' is
> definitely not suitable, because the seat has already been taken.

Well you are completely missing the point. *DON'T* abuse the modeset 
module parameters for this!

If you use 10 or any other value doesn't matter.

Regards,
Christian.

>
> Take the drm/nouveau as an example:
>
>
> ```
>
> MODULE_PARM_DESC(modeset, "enable driver (default: auto, "
>                   "0 = disabled, 1 = enabled, 2 = headless)");
> int nouveau_modeset = -1;
> module_param_named(modeset, nouveau_modeset, int, 0400);
>
> ```
>
>
> '1' is for enable the drm driver, some driver even override the 
> 'nomodeset' parameter.
>
> '2' is not suitable, because nouveau use it as headless GPU 
> (render-only or compute class GPU?)
>
> '3' is also not likely the best, the concerns is that
> what if a specific drm driver want to expand the usage in the future?
>
>
> The reason I pick up the digit '10' is that
>
>
> 1) The modeset parameter is unlikely to get expanded up to 10 usages.
>
> Other drm drivers only use the '-1', '0' and 1, choose '2' will 
> conflict with drm/nouveau.
> By pick the digit '10', it leave some space(room) to various device 
> driver authors.
> It also helps to keep the usage consistent across various drivers.
>
>
> 2) An int taken up 4 byte, I don't want to waste even a single byte,
>
> While in the process of defencing my patch, I have to say
> draft another kernel command line would cause the wasting of precious 
> RAM storage.
>
> An int can have 2^31 usage, why we can't improve the utilization rate?
>
> 3) Please consider the fact that the modeset is the most common and 
> attractive parameter
>
> No name is better than the 'modeset', as other name is not easy to 
> remember.
>
> Again, this is for Linux user, thus it is not arbitrary.
> Despite simple and trivial, I think about it more than one week.
>